1. Field of the Invention
The present invention relates to a circuit arrangement for auxiliary voltage production wherein an auxiliary voltage is produced essentially independently from a directly regulated output voltage of a transformer with the aid of a reverse-voltage which is applied to an output rectifier.
2. Description of the Prior Art
Additional auxiliary voltages are frequently required in the case of converters in order, to supply, for example an output voltage to a regulator, a monitoring circuit or a signalling unit. An essential requirement when supplying monitoring units or signalling units with an auxiliary voltage is that the auxiliary voltages should not be "hard"-coupled to the voltage of the directly regulated output circuit of the converter. If the additionally required auxiliary voltage is obtained, for example, from a further winding of either a transformer or an energy-storage inductor, it is coupled to that directly regulated circuit, that is to say that, for example, in the event of a short circuit in the directly regulated circuit, the auxiliary voltage breaks down and can no longer supply any output voltage. A reverse voltage, which is applied to AC output rectifier during a reverse phase, is used to avoid coupling the auxiliary voltage exclusively to the directly regulated output voltage.
The known circuit arrangement is illustrated in FIG. 1. The following circuit description relates to a condition when the circuit is in a steady state. The available reverse voltage U.sub.SpDio on the secondary rectifier diode GR during the reverse phase of the converter is rectified via a diode D1. Since the reverse voltage is present with low impedance on the secondary rectifier GR, a fast current riseis caused which is received by a buffer capacitor C1. In order that this current surge does not have a negative influence on, for example, any existing current mode regulation in the converter, and in order that the components used to produce the auxiliary voltage are not overloaded, this current surge is limited by a resistor R1 in series with is caused buffer or charging capacitor C1.
The capacitor C1 is now charged via this resistor R1 to the reverse voltage present on the secondary rectifier GR. If the converter is passing power to secondary side, then a voltage U.sub.A +U.sub.d is present on the secondary winding Tr1. The potential U.sub.A is in this case, the output voltage of the converter and the potential U.sub.d is the forward voltage of the diode which is used. The voltage of the secondary winding is added to the voltage on C1. The capacitor C2, which is connected in parallel with the secondary winding Tr1 and the series circuit comprising C1, D2 and R1, is charged via the resistor R1 and the diode D2. The result of this at the capacitor C2 is: EQU U.sub.H &lt;U.sub.SpDio -2Ud+U.sub.A.
In detail, the abbreviations are: PA1 U.sub.H : Auxiliary voltage, PA1 U.sub.SpDio : Reverse voltage on the secondary rectifier PA1 2Ud: Forward voltage on the diodes D1, D2 and PA1 U.sub.A : Output voltage at the converter. PA1 in the case of converters having other input voltage ranges, the losses in the resistor R1 are very high, PA1 the circuit arrangement has a high internal impedance, PA1 in principle, the circuit has low efficiency, PA1 difficult design, particularly if the converter has a wide input voltage range or there is a large load range on the directly regulated output, and PA1 if the output voltages are small, the magnitude of the auxiliary voltage which is produced is frequently inadequate.
The voltage drop across the resistor R1 which, in some circumstances, is considerable, as well as the charging time constant of C1 and C2 have not been considered here.
A disadvantage in the case of this circuit arrangement is that the auxiliary voltage U.sub.H which is produced is, in practice, generally much smaller than the theoretically stated value. Other disadvantages of the circuit arrangement previously described and illustrated in FIG. 1 can be summarized as follows: